The present invention relates to a semiconductor device that is expected to withstand high voltage, and more particularly to an effective technology applicable to a semiconductor device having a high-voltage transistor at its output stage.
A load drive circuit mounted in an automotive ECU (Electrical Control Unit) and other semiconductor device that is expected to withstand high voltage is expected to exhibit high noise immunity. For example, a low-side driver having a power transistor for driving an airbag ignition device (squib) in an automobile airbag system is expected to operate normally at a voltage not higher than an absolute maximum rated voltage (e.g., approximately 30 V). If such a low-side driver operates at a voltage higher than the absolute maximum rated voltage, it is necessary to provide protection so that, for example, the power transistor does not break down or malfunction. For example, the above-mentioned low-side driver is coupled to the squib through a harness (signal wire). However, induced current noise may be applied to the harness coupled to the low-side driver due to mutual inductance between the harness and another harness existing in an automobile so that a voltage higher than an element's breakdown voltage is applied to the output terminal of the low-side driver. As such being the case, a built-in mechanism for coupling an external protection device, such as a capacitor or a zener diode, to the output terminal of the low-side driver and performing an operation (active clamp operation) for allowing the low-side driver to absorb the current noise is incorporated to prevent the power transistor from breaking down or malfunctioning due, for instance, to the induced current noise. More specifically, an active clamp circuit is disposed between the gate and drain of a power transistor at an output stage, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-35067, so that when a voltage higher than the absolute maximum rated voltage is applied to the output terminal due to the application of significant induced current noise, the induced current noise is absorbed by raising the gate voltage of the power transistor to turn on the power transistor. Further, the external protection device coupled to the output terminal absorbs current-noise energy exceeding the absorption capacity of the power transistor to prevent the voltage at the output terminal from exceeding the breakdown voltage of the power transistor.
During an active clamp operation, the voltage at the output terminal of the low-side driver is determined by the voltage VGS between the gate and source of the power transistor and by the clamp voltage of the clamp circuit and increases with an increase in a drain current. When the voltage at the output terminal of the low-side driver reaches the breakdown voltage Bvds of the power transistor, the power transistor breaks down. Further, as electrical power increases with an increase in the time of application of current noise, the breakdown voltage decreases with a decrease in the current capacity of the power transistor (with a decrease in the transistor size). Therefore, the power transistor needs to be designed while considering the above-mentioned characteristics.